Heat transfer in near space enviroment?

I'm working on modeling the heat transfer of a small payload (approx
10" cube) covered in extruded polystyrene at high altitude (80k-100k
ft).

One thing I'm having trouble with is approximating the radiation flux
reflected back from the earth (around mid-day) to the payload, which
I'm approximating as fully incident on the bottom, and 50% on the
sides. I'm also assuming the polystyrene has a emissivity of 0.9.

With the exception of sunlight (fully incident on the top of the
payload), can I model the rest of the "sky" at 0K?

On a side note, I've looked at putting reflective materials inside the
payload, but I'm trying to prevent possible signal degradation with a
GPS reciever inside.

Any insight is greatly appreciated!

Dave

With the exception of sunlight (fully incident on the top of the
payload), can I model the rest of the "sky" at 0K?

2.7K surely!